1. Field of the Invention
The present invention relates generally to variable displacement compressors. In particular, the present invention is directed towards compressors which estimate an inclination angle of a plate of the compressor to determine a driving torque of the compressor.
2. Description of Related Art
Known variable displacement compressors may be used in an air conditioning system of vehicle. Such known variable displacement compressors include a plate, e.g., a swash plate or a cam plate, and a piston which reciprocates within a cylinder bore. An inclination angle of the plate varies in response to an external signal, and the inclination angle determines a stroke length of the piston. Specifically, when the stroke length of the piston decreases, the amount of refrigerant which the piston compresses also decreases. Similarly, when the stroke length of the piston increases, the amount of refrigerant which the piston compresses also increases. Such known variable displacement compressors also determine a driving torque of a drive shaft of the compressor, and the external signal controls the inclination angle of the cam based on the determined driving torque. Such known compressors also use the determined driving torque to control the speed of an engine of the vehicle.
In a known variable displacement compressor described in Japanese Unexamined Patent Publication No. H05-164045, the driving torque is determined by using a magnetic film wrapped around the drive shaft, and a plurality of coils positioned adjacent to the magnetic film. When the drive shaft rotates, magnetostriction occurs in the magnetic film, which alters an output voltage of the coils. The driving torque of the drive shaft then is determined based on the output voltage of the coils. Nevertheless, in this known compressor, a torsional rigidity of the drive shaft is selected, such that the drive shaft readily may be twisted. The torsional rigidity of the drive shaft may be defined as the ratio of the torque applied about a centroidal axis of the drive shaft at a first end of the drive shaft to the resulting torsional angle, when a second end of the drive shaft is fixed. However, when the torsional rigidity of the drive shaft is selected, such that the drive shaft readily may be twisted, torsional vibration may occur. Torsional vibration may be defined as a periodic motion of the drive shaft in which the drive shaft is twisted about its axis first in first direction, and then in a second direction opposite to the first direction. This periodic motion may be superimposed on the rotational motion of the drive shaft. In addition, the drive shaft is subject to a bending force, and it is difficult to manufacture a drive shaft which is both readily twistable, and has a strength which is sufficient to retain its shape against the bending force. Moreover, the use of the coils increases the size and the cost of the compressor, and if the coils are not accurately positioned within the compressor, the determined driving torque may not be sufficiently accurate.
In another known variable displacement compressor described in Japanese Unexamined Patent Publication No. H05-99156, the driving torque is indirectly determined based on a pressure within the compressor, a temperature within the compressor, or a refrigerant flow-rate within the compressor, or combinations thereof. Nevertheless, the driving torque which is determined based on these measurements also may not be sufficiently accurate.